An emission spectrum from hot diffuse gas based on the model
calculations of Mewe and Kaastra with Fe L calculations by Liedahl. The model
includes line emissions from several elements. The switch parameter determines
whether the mekal code will be run to calculate the model spectrum for each
temperature or whether the model spectrum will be interpolated from a
pre-calculated table. The former is slower but more accurate. Relative
abundances are set by the abund command for the mekal model. The vmekal
variant allows the user to set the individual abundances for the model.